1. Field of the Invention
The present invention relates generally to the field of optical waveguide fibers and passive optical component such as optical couplers, and more particularly to optical waveguide fibers used in the manufacture of fiber optic couplers.
2. Technical Background
As is well known, electromagnetic radiation traveling through an optical waveguide fiber is subject to attenuation or loss due to several mechanisms.
Such as attenuation due to absorption by the optical waveguide fiber due to impurities present in the light guiding region of the fiber. Particularly troublesome is the attenuation caused by the hydroxyl radical (OH), which can be formed in the optical waveguide fiber when a source of hydrogen is present in the fiber material, or when hydrogen available from several sources during the fiber manufacturing process diffuses into the glass. Silica bodies of the type used in optical fiber and optical fiber preform manufacture can contain a substantial amount of OH. Generally speaking, the hydrogen bonds with the oxygen available in the SiO2 and/or GeO2 and/or other oxygen containing compound in the glass matrix to form the OH and/or OH2 bonds referred to generally as “water”.
Historically, much effort has been expended to reduce the amount of water in optical fibers; until recently, telecommunications systems avoided the so-called “water peak” residing in the 1380 nm window (at which the attenuation is considerably higher than wavelengths above and below this peak) by operating in the 1310 nm window and/or the 1550 nm window, among others. With the advent of wavelength division multiplexing (“WDM”) and advancements in amplifier technology, which enable telecommunications systems to operate over broad wavelength ranges, it is now more likely that all wavelengths between about 1300 nm and about 1650 nm may be used for data transfer in optical telecommunications systems. Thus, removing water from optical waveguide fiber used with such systems has been and continues to be an important objective for enabling system operation, of higher capacity and better quality over a wider range of wavelengths, and especially for telecommunications systems which operate over long distances.
Furthermore, optical fiber networks, or optical fiber access networks, are expanding. Optical components commonly used in fiber optic devices, networks, and systems include optical couplers which may function as combiners or splitters. Couplers typically combine light propagating on at least two optical fibers, or split light propagating through an optical fiber so as to further propagate the light on at least two fibers. Optical couplers are also used for combining light of different wavelengths. In optical fiber amplifiers, couplers are typically used to feed pump light into a signal fiber. Couplers can also be used in wavelength multiplexed networks to add and drop channels.
In general, a coupler may be identified as a comprising two or more waveguides or fibers disposed in close proximity to each other wherein the mode fields of the adjacent waveguides overlap at least to some degree. Coupling can occur between two closely spaced cores or optical paths in passive optical devices. Such devices exist in the form of fused fibers, overclad fused fibers, planar devices and the like.
Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in one or two optical fibers is coupled to or switched between two output fibers. Possible commercial embodiments of such 1×2 and 2×2 fiber optic couplers include devices such as 3 dB couplers, achromatic couplers, taps, wavelength division multiplexers (WDMs), switches and the like.
Wavelength division multiplexing is employed in fiber optic communication systems and local area networks to increase data transmission capacities. Various wavelength multiplexer/demultiplexers are available, such as high bandwidth, low loss single-mode fiber telemetry systems.
WDM couplers are used for the splitting of wavelengths. WDM couplers may be manufactured using a process called Fused Biconical Taper (FBT) technology. An FBT fiber coupler comprises two or more optical fibers whose optical claddings have been fused together. The fibers are heated and tapered by elongation until desired coupling properties are achieved for allowing the transfer of light between the fibers.
The Erbium Doped Fiber Amplifier (EDFA), which operates at an operating window around a 1550 nm wavelength, is an important component in long haul fiber optic network. The EDFA amplifies transmission signals by use of a 980 or 1480 nm pump laser. The pump wavelength (980 or 1480 nm) is introduced to excite the erbium doped in a piece of fiber, so that as a transmission signal in the 1550 nm window passes through the fiber, the signal is boosted and exits the amplifier. WDM couplers are commonly used in such amplifiers to combine pump and signal wavelength(s), thereby enabling amplification of the signal wavelength(s).